Regenerator for a wavelength division multiplex transmission system

ABSTRACT

The invention relates to wavelength division multiplex fiber optic transmission systems. It proposes a regenerator including a demultiplexer for separating the signals of various channels, a plurality of optical modulators each receiving signals from the demultiplexer and a modulation clock from a clock distribution unit, and a multiplexer combining the signals modulated by the modulators. The clock distribution unit includes a reference clock and, for each modulator, means for synchronizing the phase of a copy of the reference clock with the signals applied to the modulator. A phase-locked loop can be used for phase synchronization, with a phase shifter controlled in accordance with the average power of the signals at the output of the modulator. The invention enables only low-frequency components to be used to generate modulation clocks from a single reference clock.

[0001] The invention relates to wavelength division multiplex fiberoptic transmission systems and more precisely to regenerating signals byoptical modulation in wavelength division multiplex fiber optictransmission systems.

BACKGROUND OF THE INVENTION

[0002] It has been proposed to make regular use in wavelength divisionmultiplex fiber optic transmission systems of periodic regeneration ofsignals by synchronous modulation. The modulation is preferably optical,especially in high bit rate systems. The problem then arises of groupvelocity differences between the channels due to the differences betweenthe wavelengths and the dispersion of the line optical fibers. The groupvelocity differences desynchronize the bit times of the various channelsand prevent simultaneous synchronous modulation of the channels of themultiplex.

[0003] A first solution to the problem is to separate the variouschannels before using synchronous modulation to regenerate each channel.The regenerated channels can then be multiplexed. A regenerator in thatconfiguration is made up of the same number of synchronous modulators asthere are channels, connected in parallel. Because the channels are notsynchronized each regenerator has its own clock recovery circuit. Thatsolution is bulky, costly and has a high power consumption because ofthe number of electronic circuits to be replicated for clock recovery.

[0004] French Patent Application 99 02 126 filed on Feb. 2, 1999 (Fo.102078) proposes to regenerate only a subset of channels in aregenerator, for example one channel in four. All channels are processeddownstream of a plurality of regenerators, four regenerators in thatexample. That solution reduces the number of modulators to be connectedin parallel in each regenerator, by a factor of four in that example.For the same performance, the total number of modulators in thetransmission system remains the same. That solution simplifies thestructure of a regenerator but has no effect on cost or powerconsumption.

[0005] Other solutions to the problem of synchronous regeneration entailsynchronizing the various channels periodically in order to regeneratethe channels simultaneously at the point of synchronicity. An article byE. Desurvire, O. Leclerc and O. Audouin entitled “Synchronous in-lineregeneration of wavelength division multiplexed solitons signals inoptical fibers”, Optics Letters, vol. 21, no. 14, pages 1026-1028,describes a scheme for allocating wavelengths that is compatible withthe use of synchronous modulators for soliton signals. The articleproposes allocating wavelengths to the various channels of the multiplexso that, for given intervals Z_(R) between repeaters, the signals of thevarious channels, or to be more precise the bit times of the variouschannels, of the multiplex are substantially synchronized on reachingthe repeaters. That enables in-line synchronous modulation of allchannels at given intervals using discrete synchronous modulators. Thattechnique of allocating the wavelengths of the multiplex is alsodescribed in French Patent Application FR-A-2 743 964 in the name ofAlcatel Submarine Networks. The article proposes choosing a sub-group ofchannels that are synchronous not only with intervals Z_(R) but alsowith intervals which are sub-multiples of Z_(R).

[0006] An article by O. Leclerc, E. Desurvire and O. Audouin entitled“Synchronous WDM soliton regeneration: towards 80-160 Gbit/stransoceanic systems”, Optical Fiber Technology, 3, pages 97-116 (1997),specifies that the above wavelength allocation scheme can lead toexcessively large intervals Z_(R) between the synchronous modulators orto excessively large spacings between the channels of the multiplex. Toalleviate that problem, the article notes that in that kind ofwavelength allocation scheme the bit times of the subsets of channels ofthe multiplex are synchronous with intervals that are sub-multiples ofZ_(R). The article consequently proposes regenerating subsets ofchannels of the multiplex at smaller intervals. However, that solutionimposes filtering of the channels of the subset to be regenerated andthe transmission system loses the benefit of a single period for allchannels.

[0007] FR-A-2 770 001 proposes using a synchronous modulator to modulatethe soliton signals of all the channels of a wavelength divisionmultiplex transmission system at a frequency N/T which is a multiple ofthe clock frequency 1/T of the signals. That loosens the synchronicityconstraint by requiring the various channels to be synchronized to asub-multiple of the bit time, instead of imposing synchronization of thebit times.

[0008] FR-A-2 759 516 proposes demultiplexing the various channels andapplying the necessary time-delays to them to resynchronize them. Aftermultiplexing, the various channels can be modulated by a single opticalmodulator. French Patent Application 97 06 590 proposes using a chain ofgratings formed in a fiber to apply suitable time-delays and therebyresynchronize the various channels without having to employdemultiplexing and remultiplexing. An article by M. Nakazawa et al.entitled “160 Gbit/s WDM (20 Gbit/s×8 channels) soliton transmissionover 10 000 km using in-line synchronous modulation and opticalfiltering”, Electronics Letters, vol. 34, no. 1 (1998), pp. 103-104,proposes resynchronizing the channels of a wavelength division multiplexusing a different dispersion compensating fiber (DCF) for each channel.The article proposes using four modulators to regenerate eight channels,three of the modulators processing two channels simultaneously.

[0009] The above solutions have the following drawbacks. The passivesolutions (choice of wavelengths or application of fixed time-delays)have the drawback of high sensitivity to the characteristics of thetransmission system and in particular to dispersion, temperature, andregenerator positioning. There is additionally the problem of aging ofthe systems and that of correlated variations in the abovecharacteristics.

OBJECTS AND SUMMARY OF THE INVENTION

[0010] The invention proposes a solution to the problem of regeneratingchannels of wavelength division multiplex transmission systems. Itreduces the cost and power consumption of the regenerators withoutimposing any constraint on the choice of wavelengths.

[0011] To be more precise, the invention proposes a regenerator for awavelength division multiplex transmission system, including ademultiplexer adapted to separate the signals of various channels, aplurality of optical modulators each adapted to receive signals from thedemultiplexer and a modulation clock from a clock distribution unit, anda multiplexer adapted to combine the signals modulated by saidmodulators, in which regenerator the clock distribution unit includes areference clock and, for each modulator, means for synchronizing thephase of a copy of the reference clock with the signals applied to themodulator.

[0012] In one embodiment, the phase synchronization means include aphase-locked loop for each modulator.

[0013] In this case the phase-locked loop includes a phase shifterreceiving a copy of the reference clock and supplying a modulation clockand the phase shifter is controlled in accordance with the average powerof the output signals of the modulator.

[0014] The phase-locked loop preferably includes a coupler adapted tosample a portion of the output signals of the modulator and a photodiodeadapted to receive the signals from the coupler and to supply a voltagerepresentative of the average power of the output signals of themodulator.

[0015] In this case the phase shifter is controlled by a signal that isa function of the difference between said voltage and a referencevoltage.

[0016] The reference voltage can depend on the total power of thesignals at the output of the regenerator or can be remote-controlled.

[0017] In another embodiment of the invention, the reference clock issupplied by a voltage-controlled oscillator. In particular, it can becontrolled as a function of the signals applied to the regenerator.

[0018] In a further embodiment the regenerator includes a coupler forsampling a portion of the input signals of the regenerator and a clockrecovery circuit adapted to receive signals sampled by the coupler andto supply at its output a control signal for the oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Other features and advantages of the invention will becomeapparent on reading the following description of embodiments of theinvention, given by way of example and with reference to theaccompanying drawings, in which:

[0020]FIG. 1 is a diagram showing a regenerator of the invention;

[0021]FIG. 2 is a diagram showing a clock distribution unit of theregenerator shown in FIG. 1; and

[0022]FIG. 3 is a diagram showing a different embodiment of the clockdistribution unit of the regenerator shown in FIG. 1.

MORE DETAILED DESCRIPTION

[0023] To reduce the volume, cost, and complexity of regenerators, theinvention proposes sharing a common clock distribution unit between thesynchronous modulators. The clock distribution unit has a referenceclock which is replicated to provide as many copies as necessary. Eachcopy is synchronized in phase with one channel. A phase-locked loop canbe used to synchronize the phase of the copies of the reference clockand the corresponding channels.

[0024] The remainder of the description refers to an embodiment of aregenerator in a transmission system with four channels. The inventionis not limited to this number of channels, of course. In the example,the four channels are modulated separately. The invention appliesequally well to the situation described in the M. Nakazawa article inwhich some channels are modulated by the same modulator.

[0025]FIG. 1 is a diagram showing a regenerator of the invention. Theregenerator includes a demultiplexer 2 which receives the multiplexedsignals of the various channels at its input and supplies the separatedchannels at its output. The channels supplied at the output of themultiplexer are applied to a set of optical modulators. The variousmodulators can be combined on a single InP chip 4. This is known in theart. The chip has clock inputs 6 which receive the clocks for modulatingthe various channels, inputs 8 which receive the signals of theseparated channels from the demultiplexer 2, and outputs 10 which supplythe signals of the various channels modulated by the clocks. A coupler12 _(i) at the output of the modulators of each channel samples aportion of the modulated signals. The modulated signals are thencombined in a multiplexer 14.

[0026] The regenerator also includes a clock distribution unit 16 whichreceives signals A_(i) sampled at the output of the modulator of eachchannel and supplies clocks B_(i) to each modulator. In the invention,the clock B_(i) for a channel λ_(i) is obtained by locking the phase ofa reference clock on the basis of the signal sampled at the modulatoroutput for the channel in question.

[0027] The FIG. 1 regenerator operates as follows. The signals of thevarious channels are separated in the multiplexer 2, modulated byrespective modulators and then recombined in the multiplexer 14. Theclock distribution unit supplies the clocks needed for modulation. Theoperation of the clock distribution unit is described in more detailhereinafter.

[0028] The invention enables sharing of the clock distribution unit andavoids the multiplicity of circuits used in prior art regenerators. Theregenerator also locks the phase of the modulation clocks for eachchannel independently of variations in the phase of the various channelson transmission. The regenerator is also insensitive to output powervariations, due in particular to location in the transmission system,equipment failures or interruptions of transmission on the variouschannels.

[0029]FIG. 2 is a diagram showing the clock distribution unit of theFIG. 1 regenerator. As indicated with reference to that figure, thedistribution unit receives signals A_(i) sampled downstream of themodulators and supplies clocks B_(i). The signals sampled downstream ofthe modulators are fed to low-frequency photodiodes 16 _(i) which supplyrespective voltages representative of the average power of the modulatedsignal of each channel. In the expression “low-frequency photodiode”,the term “lowfrequency” refers to frequencies below the frequency of thesignals transmitted. Signals at 10 Gbit/s on each channel are typicallyconsidered in this example. The photodiode can be a photodiode whosefrequency response levels off at a few tens or hundreds of kHz. Aphotodiode therefore supplies a signal whose power is representative ofthe average power of the modulated signal of a channel over a windowwith a length of the order of a few tens of thousands or a few millionsof bits.

[0030] The voltage supplied by each photodiode is applied to thenon-inverting input of a low-frequency amplifier 18 _(i) which isconfigured as a comparator. In the expression “low-frequency amplifier”,the term “low-frequency” means, as with the photodiodes, that thefrequency response of the amplifier levels off at a few tens or hundredsof kHz, for example. Each amplifier receives a reference voltage V_(REF)at its inverting input and supplies at its output a voltage C_(i)representative of the difference between the output voltage of thephotodiode and the reference voltage.

[0031] The output voltage of each amplifier 18 _(i) is fed to a controlinput of a phase controller 20 _(i) shown in the figure as a phaseshifter Φ_(i). Phase controllers or shifters of this kind arecommercially available or can be implemented using various types ofgeneric electronic circuit: analogue/digital, hybrid/integrated, etc.Each phase controller also receives a reference clock and supplies atits output a clock B_(i) derived from the reference clock and whosephase depends on the voltage applied to its control input. In the FIG. 2embodiment, the reference clock is supplied by a voltage-controlledoscillator (VCO) 22. The clock is adjusted to the frequency of thesignals transmitted on each channel, which is 10 GHz in this example.The reference clock can be adjusted by varying the voltage applied tothe oscillator, either at installation time or in use. FIG. 3 shows aclock recovery circuit for adjusting the voltage applied to theoscillator.

[0032] The FIG. 2 clock generation unit operates as follows: the phaseof the reference clock applied to each phase controller 20 _(i) isadjusted in the phase-locked loop formed by the phase controller 20_(i), the modulator, the photodiode 16 _(i) and the amplifier 18 _(i).For a given channel, the operation of the phase-locked loop is based onthe fact that the average power of the modulated signal is at a maximumwhen the modulation clock B_(i) is in phase with the signal. The outputvoltage of the photodiode 16 _(i) varies between a minimum value for aphase difference of π between the modulation clock and the signals to bemodulated and a maximum value when the modulation clock and the signalsto be modulated are in phase. The ratio between the minimum value andthe maximum value depends on the chosen depth of modulation, a phasedifference of π “crushing” the pulses by an amount equal to themodulation depth. The maximum value depends on the proportion of “0” and“1” bits in the signals to be modulated and on the characteristics ofthe photodiodes. As explained in more detail below, the referencevoltage V_(REF) is preferably equal to this maximum value.

[0033] Accordingly, for a random value of the phase of the controlsignal B_(i), the output of the modulator has an average power whichdepends on the phase and the output voltage of the photodiode and isbetween zero and the reference voltage V_(REF). If the output voltage ofthe photodiode is zero, the differential voltage supplied by theamplifier 18 _(i) is non-zero and the phase controller 20 _(i) thereforevaries the phase of the clock B_(i). When the phase of the clock B_(i)approaches the phase of the signals to be modulated, the voltagesupplied by the photodiode approaches the reference voltage V_(REF) andthe control voltage applied to the phase controller 20 _(i) approacheszero. The loop therefore locks the phase of the clock B_(i) to the phaseof the signals to be modulated in the corresponding channel.

[0034] The use of low-frequency components (photodiodes and differentialamplifiers) limits the cost of the clock distribution unit of theinvention. Less costly ordinary electronic components can be used. Inparticular, it is not necessary to use in the FIG. 1 and 2 embodiment ahigh-speed photodiode, i.e. a device that is capable of supplying asignal varying substantially with the frequency of the signalstransmitted. The clock distribution unit can be implemented on a singlemicrochip by integrating components (photodiodes, amplifiers, phasecontrollers) using Si or S—Ge technology.

[0035] As indicated above, the reference voltage V_(REF) is preferablyequal to the maximum voltage supplied by the photodiode when the clockfor the corresponding channel is in phase with the signals to bemodulated. Small variations between the maximum voltage of thephotodiodes and the reference voltage are acceptable, however. Sucherrors can also be corrected by generic control devices that are morecomplex than the solution proposed hereinafter, in particular throughcoupling with supervisory signals. In this case, the reference voltagecould be remote-controlled in accordance with the results ofsupervision. This solution enables adjustment of the reference voltagein accordance with power fluctuations or imperfections of the device.

[0036] A solution for equalizing the maximum voltage supplied by thephotodiodes and the reference voltage consists of slaving the referencevoltage to the average voltage of the modulated signals, as explainedwith reference to FIG. 3, which is a diagram showing a differentembodiment of the clock distribution unit. FIG. 3 shows, for only onechannel, the components already described with reference to FIGS. 1 and2. In this embodiment, the regenerator includes a coupler 24 whichsamples a portion of the modulated signals at the output of themultiplexer 14. The signals are applied to a low-frequency photodiode 26of the same kind as the photodiode 16 _(i) and which supplies a voltagerepresentative of the average power of the signals of the variouschannels. That voltage is applied to a voltage divider 28 which alsoreceives a control voltage V_(c) and supplies at its output a voltagerepresentative of the average power of the signals in a channel. Thedivision ratio of the voltage divider 28 depends on the number ofchannels. The control voltage V_(c) can advantageously beremote-controlled. It is then possible to compensate remotely forvariations in the number of channels, for example in the event ofinterruption of transmission on a channel for repairs or following anincident.

[0037] A feedback loop could also be used to adjust the referencevoltage, having a time constant greater than the phase-locked loop usedto adjust the phases of the modulation clocks so as to be sure that thereference voltage remains substantially constant even if the phases ofthe clocks vary in time.

[0038]FIG. 3 also shows an embodiment in which the regenerator furtherincludes clock recovery means. The regenerator includes upstream of thedemultiplexer 2 a coupler 30 for sampling a portion of the multiplexedsignals. The sampled signals are fed to an optical filter 32 whichfilters out the signals of a reference channel. The filtered signals areapplied to a clock recovery circuit 34 which supplies at its output asignal representative of the clock frequency of the reference channel.That signal can be used to control the voltage-controlled oscillator.The clock recovery circuit could also be used instead of an oscillator,supplying the reference clock directly and taking as its own referencean optical signal sampled in the multiplex.

[0039] The regenerator of the invention applies to all wavelengthdivision multiplex transmission systems. It regenerates channelsoptically, without the constraints of prior synchronization, and withoutincreasing the number of components needed to generate the variousmodulation clocks.

[0040] Of course, the present invention is not limited to the examplesand embodiments described and shown, but is open to many variants thatwill be evident to the skilled person. The examples consider thesimplest case, in which the channels are modulated separately. Asmentioned above, some modulators or all the modulators could modulatemore than one channel. Solutions other than a voltage-controlledoscillator can be used to generate the reference clock, for example afixed-frequency oscillator or the output of a clock recovery circuitslaved to a reference channel extracted from the multiplex. Nor does thefigure show components such as filters or amplifiers that can be addedto a regenerator.

1. A regenerator for a wavelength division multiplex transmissionsystem, including a demultiplexer adapted to separate the signals ofvarious channels, a plurality of optical modulators each adapted toreceive signals from the demultiplexer and a modulation clock from aclock distribution unit, and a multiplexer adapted to combine thesignals modulated by said modulators, in which regenerator the clockdistribution unit includes a reference clock and, for each modulator,means for synchronizing the phase of a copy of the reference clock withthe signals applied to the modulator.
 2. The regenerator of claim 1 ,wherein the phase synchronization means include a phase-locked loop foreach modulator.
 3. The regenerator of claim 2 , wherein the phase-lockedloop includes a phase shifter receiving a copy of the reference clockand supplying a modulation clock and the phase shifter is controlled inaccordance with the average power of the output signals of themodulator.
 4. The regenerator of claim 3 , wherein the phase-locked loopincludes a coupler adapted to sample a portion of the output signals ofthe modulator and a photodiode adapted to receive the signals from thecoupler and to supply a voltage representative of the average power ofthe output signals of the modulator.
 5. The regenerator of claim 4 ,wherein the phase shifter is controlled by a signal in accordance withthe difference between said voltage and a reference voltage.
 6. Theregenerator of claim 5 , wherein the reference voltage depends on thetotal power of the signals at the output of the regenerator.
 7. Theregenerator of claim 5 , wherein the reference voltage isremote-controlled.
 8. The regenerator of claim 1 , wherein the referenceclock is supplied by a voltage-controlled oscillator.
 9. The regeneratorof claim 8 , wherein the oscillator is controlled in accordance with thesignals applied to the regenerator.
 10. The regenerator of claim 8 ,including a coupler for sampling a portion of the input signals of theregenerator and a clock recovery circuit adapted to receive signalssampled by the coupler and to supply at its output a control signal forthe oscillator.
 11. A wavelength division multiplex transmission systemincluding a regenerator according to claim 1 .